1. Field of the Invention
The present invention relates to an optical recording medium having space therein and a method of manufacturing the same.
2. Related Background Art
In recent years, extensive studies and developments have been made for information recording media for optically recording/reproducing information in favor of high-density, high-capacity recording, random access, and low cost per bit. A sectional structure of a typical example of a conventional optical recording medium (two-sided recording type) having a hollow structure is illustrated in FIG. 1. Disk-like substrates 1a and 1b central holes transparent to a laser beam, and are made of glass, a plastic or the like. Optical recording layers 2a and 2b are respectively formed on the substrates 1a and 1b. The recording layers 2a and 2b are made of a material subjected to a shape change, a phase change, a magnetooptical change, a chemical decomposition, or the like upon laser radiation. An auxiliary layer such as a very thin reflecting layer or a protective layer may be formed on each recording layer. The substrates 1a and 1b respectively having the recording layers 2a and 2b thereon are adhered by ring-like adhesive layers 3 and 4 respectively defining the inner and outer peripheral portions of the substrates 1a and 1b, so that adhesive layers 3 and 4 serve as spacers to define a space 5.
In the conventional optical recording medium having a hollow structure described above, the thicknesses of the adhesive layers serving as the spacers must be uniform. If variations in distance between the substrates 1a and 1b defining the space 5 occur, vertical displacements of the surface of the recording disk occur upon its rotation on a turntable. It is therefore difficult to appropriately perform focusing control and tracking control.
In order to set the distance between the substrates 1a and 1b uniform, Japanese Pat. Laid-Open Application No. 226046/1985 describes an optical recording medium, the sectional structure of which is illustrated in FIG. 2. The medium shown in FIG. 2 does not have a hollow structure. Substrates 1a and 1b having recording layers 2a and 2b thereon are adhered by an adhesive 6 prepared by dispersing spacing members 7 therein, so that; the entire inner surfaces of the substrates 1a and 1b are in contact with the adhesive 6. With the above structure, the distance between the substrates 1a and 1b is kept uniform according to the thickness of the spacing members 7. The spacing member 7 is made of glass or a resin material such as polymethyl methacrylate (PMMA). An ultraviolet curing adhesive is preferably used as the adhesive 6. Japanese Pat. Laid-Open Application No. 168947/1984 also discloses an example wherein the above technique is applied to an optical recording medium having a hollow structure. This example will be described with reference to the sectional view of FIG. 3. Referring to FIG. 3, the inner and outer peripheral portions of disk-like substrates 1a and 1b having recording layers 2a and 2b thereon are adhered by ring-like adhesive layers 13 and 14 serving as spacers, respectively. The adhesive layers 13 and 14 are made of an adhesive 15 in which granular or filler type spacing members 16 are dispersed. The size of a space 6 between the substrates 1a and 1b is kept uniform according to the thickness of the spacing member 16.
With the above structure, however, a nonuniform distribution of the spacing members between the substrates 1a and 1b often fails to keep the distance between the substrates 1a and 1b uniform with rated high precision. For example, when adhesive layers are to be partially formed between the substrates 1a and 1b, a mixture of spacing members and an adhesive is stored in a tank and is supplied in a small amount in each operation cycle. In this case, a mixing ratio of the adhesive to the spacing members varies as a function of time, thus causing a nonuniform distribution of the spacing members.